The present invention relates to a froth flotation method of removing carbon from fly ash, and more particularly, to an improved method which utilizes an environmentally friendly conditioning agent to aid in removing carbon from the fly ash.
Fly ash is a combustion residue generated from utilities which burn coal, oil, or other solid fuels. It consists of fine particles of various mineral matters and unburned carbon. Electrostatic precipitators, cyclones, and baghouses are commonly used to collect fly ash particles from combustion emissions. The chemical composition and particle size distribution of fly ash varies widely, depending on the source of fuel, fuel preparation conditions, combustor, and combustion conditions. The mineral components are mostly silicates, comprising oxides of silicon, aluminum, iron, calcium, magnesium, potassium, sodium and others, but can also include metal oxides such as vanadium oxide.
The carbon content of fly ash usually varies from about 0.5 to 20%, although carbon contents of up to 50% have been reported. The content varies, depending on combustion efficiency. In order to meet the low nitrogen oxide emissions requirement, many utilities have decreased the combustion temperature and/or oxygen supply during combustion, which has resulted in fly ash having a higher carbon content. However, the high carbon content in fly ash has frequently limited the use of fly ash in concrete applications as the carbon adsorbs air entraining agents in concrete and subsequently decreases the air content of concrete, which can cause the concrete to crack easily during freeze and thawing cycles. Therefore, it is desirable to separate carbon from fly ash for most applications.
Froth flotation has been found to be an effective method for the separation of carbon from fly ash. In a typical froth flotation system, fly ash is mixed with water to form a slurry and a flotation reagent such as oil is then added to the slurry. While mixing, oil droplets are adsorbed and coat the carbon particles and render them hydrophobic. Air is then introduced to the slurry in a froth flotation machine, and air bubbles attach to the hydrophobic particles and carry them to the surface of the slurry, which is then skimmed off such that the carbon is separated.
The selection of flotation reagents is critical for the froth flotation separation. Many different oils have been proposed for use as froth flotation reagents. For example, U.S. Pat. No. 4,121,945 discloses the use of kerosene oil; U.S. Pat. No. 4,426,282 discloses the use of mineral oil, U.S. Pat. No. 5,047,145 discloses the use of fuel oil, and U.S. Pat. No. 5,456,363 discloses the use of a mixture of fuel oil and petroleum sulfonate. All of these patents are hereby incorporated by reference generally with regard to froth flotation processes and equipment. However, the oils disclosed in these patents are not environmentally friendly as they are all petroleum based and are not easily biodegradable.
Accordingly, there is still a need in the art for an environmentally friendly method of removing carbon from fly ash.